14. PRELIMINARY ASSESSMENT OF ORGANIC ...PRELIMINARY ASSESSMENT OF ORGANIC GEOCHEMICAL SIGNALS IN SEDIMENTS FROM HOLE 893A, SANTA BARBARA BASIN, OFFSHORE CALIFORNIA 1 K.-U. Hinrichs,

Kennett, J.P., Baldauf, J.G., and Lyle, M. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 146 (Pt. 2)

14. PRELIMINARY ASSESSMENT OF ORGANIC GEOCHEMICAL SIGNALS IN SEDIMENTS FROMHOLE 893A, SANTA BARBARA BASIN, OFFSHORE CALIFORNIA1

K.-U. Hinrichs,2 J. Rullkötter,2 and R. Stein3

ABSTRACT

Preliminary results of molecular organic geochemical analyses of 26 samples from Hole 893A are reported in this paper.Detailed lipid analyses of 10 selected samples show a complex molecular composition of marine, terrestrial, and bacterial ori-gin. Numerous steroidal alcohols of predominantly autochthonous origin form the major compound class in the bitumen.Higher plant-derived long-chain rc-fatty acids, rc-alcohols, and «-alkanes are also present in significant concentrations. Thehydrocarbon fractions of all investigated samples contain a series of 17α(H)-hopanes that show a signature typical of biode-graded petroleum. These compounds are derived from eroded Monterey Formation petroleum source rocks or related oil seeps,or—considered less likely—from local migration of oil from greater depths. Several molecular parameters appear to distin-guish laminated from nonlaminated sediments.

INTRODUCTION

The assemblage of organic matter components in marine sedi-ments is controlled by a number of factors, including the primary au-tochthonous productivity, the supply of allochthonous organic matterfrom the continent (terrestrial plant material or eroded rock), preser-vation during and after deposition, redeposition of material on sub-aquatic slopes, diagenetic alteration within the sediment, andmigration of mobile components from deeper sediment layers. Mostof the predepositional factors will be influenced by climatic and re-lated oceanographic changes (sea level, currents) and these mayeventually be reflected in the fossil organic matter composition.Drilling of recent and subrecent sediments in the Santa Barbara Basinby the Ocean Drilling Program (ODP) provided the opportunity toaddress this subject.

Site 893 is located at 34° 17.25 'N, 120°02.2'W, in the center of theSanta Barbara Basin on the southern Californian continental margin(Fig. 1). Drilling during Leg 146 recovered 196.5 m of sediment witha relatively uniform lithology, with the main variation being the ex-tent of lamination. The sequence represents a sedimentary record ofthe last 160,000 yr (Fig. 2; Kennett, Baldauf, et al., 1994). The basinis semi-enclosed and has a maximum water depth of about 600 m.The sill to the open Pacific Ocean is at a water depth of 475 m and atpresent lies within the East Pacific Oxygen Minimum Zone. This re-sults in suboxic bottom-water masses in the central basin below thatdepth (Sholkovitz and Gieskes, 1971; Kennett, Baldauf, et al., 1994).The high biological productivity in the surface water of the CaliforniaBorderland is responsible for the accumulation of organic carbon-rich anoxic muds in the Santa Barbara Basin at high sedimentationrates. In the Holocene central basin, annual pairs of light and darklayers are induced by seasonal variation of marine and terrigenoussediment supply, growth of bacterial mats, and bottom-water redoxconditions (e.g., Soutar and Crill, 1977; Pisias, 1978; Reimers et al.,

'Kennett, J.P., Baldauf, J.G., and Lyle, M. (Eds.), 1995. Proc. ODP, Sci. Results,146 (Pt. 2): College Station, TX (Ocean Drilling Program).

Institut für Chemie und Biologie des Meeres (ICBM), Carl von Ossietzky Univer-sitàt Oldenburg, Postfach 2503, D-26111 Oldenburg, Federal Republic of Germany.

• Alfred-Wegener-Institut für Polar- und Meeresforschung, Columbusstrasse, D-27568 Bremerhaven, Federal Republic of Germany.

1990; Schimmelmann and Tegner, 1990; Schimmelmann and Kast-ner, 1992).

The knowledge of the sedimentary history of the Santa BarbaraBasin until now is restricted to the Holocene section. The results ofhigh-resolution studies of varve chronology, sedimentology, stableoxygen isotopes, microfossil assemblages, and organic matter com-position were interpreted in terms of changes in precipitation, terrig-enous sediment supply, surface-water temperatures, primaryproductivity, intensity of the California Current, and oxygen level ofthe water masses by (e.g., Fleischer, 1972; Soutar and Crill, 1977;Heusser, 1978; Pisias, 1978;Dunbar, 1983; Lange et al., 1990; Bern-hard and Reimers, 1991; Kennedy and Brassell, 1992a, 1992b).

The analysis of lipids at the molecular level so far is essentiallyrestricted to the studies of Kennedy and Brassell (1992a, 1992b).They concentrated on the analysis of long-chain alkenones in order toestablish a sea-surface temperature record with an approximately an-nual resolution. In addition, they investigated concentration profilesof biomarkers of marine origin (dinosterol, brassicasterol, and cho-lesterol) down a shallow hole, which were interpreted to reflect pro-ductivity trends. They related their results to historical climaticrecords such as sea-surface temperature measurements and reportedEl Nino events and found a good correlation between geochemicaland historical records.

Heath et al. (1977) performed a high-resolution study of total or-ganic carbon (TOC) contents of Santa Barbara Basin sediments fromthe last 10 ka in order to compare the diagenetic degradation rates ofterrestrial and marine organic material. They found that the degrada-tion of marine organic material occurs 3.5 times faster than that ofterrigenous material.

Schimmelmann and Tegner (1990) and Schimmelmann and Kast-ner (1992) investigated the history of laminated sediments of the last1000 yr on the basis of organic carbon contents, composition of re-duced sulfur compounds, and stable carbon isotopes of bulk organicmaterial. The diagenesis of sulfur compounds occurs mainly duringthe first 500 yr after deposition. Significant peaks in isotopic recordscorrespond to historical El Nino events and strong storms.

Site 893 provides the opportunity to study the influences of long-er-term climatic changes (glacial-interglacial cycles) on the sedimen-tary record of the Santa Barbara Basin. Sea-level changes control thetransport mechanisms of terrestrial mineral and organic matter to the

K.-U. HINRICHS, J. RULLKOTTER, R. STEIN

120° 30'I

34° 30' -

34° 00' -

- 34° 30'

-34° 00'N

120° 30' 120° 00' 119°30'W

Figure 1. Map of the Santa Barbara Basin and location of Site 893 (from Kennett, Baldauf, et al., 1994).

central basin, and they may alter the marine primary productivity inthe water column. Sea-level variations may also change the redoxconditions at the sediment/water interface in the central Santa Bar-bara Basin, which greatly influence the rate of preservation of organ-ic material and the development of benthic activities including thegrowth of bacterial mat communities.

This paper reports the preliminary results of a molecular study oflipid composition on a selected set of samples, using gas chromato-graphic (GC) and gas chromatographic/mass spectrometric (GC/MS)methods.

We focused our investigations on the following questions:

What are the differences in molecular composition between lam-inated and homogeneous sediments in terms of provenanceand preservation of organic matter, particularly with respect tothe importance of bacterial organic matter?

Are there similarities or differences in the lipid composition ofsediments deposited under similar sea-level conditions (e.g.,Holocene vs. isotopic Stage 5e)?

Are there any biomarkers that are specific for particular paleoen-vironmental (climatic) conditions in the Santa Barbara Basin?

The results are related to a high-resolution study of bulk organicmatter characteristics that was performed by Stein and Rack (this vol-ume).

EXPERIMENTAL METHODS

We investigated 26 core samples from Hole 893A (Fig. 2) ofwhich 10 were selected for detailed studies of the molecular organicmatter composition. After freeze-drying and grinding, the sediments

were analyzed for their TOC, carbonate, and sulfur contents by com-bustion in a LECO CS-444 instrument. The release of hydrocarbonsduring temperature-programmed pyrolysis (Espitalié et al., 1977)was determined on a Rock-Eval II instrument (Geocom); results arepresented as hydrogen indices (HI: mg hydrocarbons per g TOC).Solvent extractions were performed in a Soxhlet apparatus withdichloromethane plus 1 % methanol over 48 hr.

Separation into fractions of different polarities was carried out on80% of the total extract of each sample after addition of internal stan-dards (squalane, erucic acid [n-C22:\], 5α-androstan-17-one). Theyield of each fraction was determined gravimetrically. Prior to sepa-ration, the hexane-insoluble fraction (asphaltenes) was precipitated.

The hexane-soluble portion was separated by medium-pressureliquid chromatography (MPLC; Radke et al., 1980) into fractions ofaliphatic hydrocarbons, aromatic hydrocarbons, and polar com-pounds (NSO fraction). Subsequently, carboxylic acids were separat-ed from the NSO fraction by using a column filled with KOH-impregnated silica gel (63-200 µm; a solution of 0.5 g KOH in 10 mLjso-propanol was added to 4 g silica gel). The nonacidic compoundswere eluted from the column with 120 mL of dichloromethane. Fol-lowing this, 120 mL of a solution of formic acid (2% in dichlo-romethane) transformed the potassium salts of the acids back to theoriginal acids. The acid-free NSO fraction then was separated intoketones and alcohols by flash chromatography (Still et al., 1978) un-der a moderate overpressure of nitrogen. A 10-mm × 200-mm col-umn was filled with 5 g silica gel 60 (40-63 µm, deactivated with 5%by weight of water) and washed with 50 mL of dichloromethane. An-other 60 mL of dichloromethane was used to elute the fraction of ke-tones at a rate of two drops per second. The alcohols were removedfrom the column with 50 mL of a mixture of dichloromethane andmethanol (10% by volume). For molecular studies, the acid fractionwas methylated with diazomethane, and ketone and alcohol fractions

202

ORGANIC GEOCHEMICAL SIGNALS IN SEDIMENTS

10 -

20 -

30 -

40 -

50 -

60 -

70 -

80 -

90 -

100

1H

2H

3H

4H

5H

6H

7H

8H

9H

10H

11H

1H

2H

3H

4H

5H

6H

7H

8H

σ><

ΦCCDü_OO

X

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JP.Q_Φ

-*—•

24.3

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CD

σ>

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CO

[V] Nannofossils [ ^ Diatoms (OOj Silty clay / clayey silt

|^3 Foraminifers ^ Laminations ^ Sand ^ Gray clayey silt / silty clay

Figure 2. Lithostratigraphy of Holes 893A and 893B (from Kennett, Baldauf, et al., 1994). Arrows indicate origin of samples used for molecular organicgeochemical bitumen analysis in this study.


Table 1. Bulk organic carbon data, carbonate contents, extract yields, U k values, and void-corrected depths for Hole 893A sediments.

Core, section,interval (cm)

146-893A-1H-1, 114-1182H-3, 109-1133H-4, 125-1304H-4, 142-1465H-5, 113-1186H-3, 32-366H-6, 4 3 ^ 87H-5, 22-278H-3, 75-809H-2, 47-5210H-3, 65-7011H-4, 96-10112H-2, 29-3413H-7, 131-13514H-4, 64-6915H-2, 77-8215H-6, 27-3216H-1, 55-5916H-7, 50-5517H-4, 70-7518H-3,40^419H-2, 45-5019H-3, 94-9920H-2, 108-11221H-1, 84-8821H-3.7-12

Depth*(mbsf)

1.1410.5121.6630.7441.5247.2251.5657.7865.6474.6986.0097.26

103.12119.36125.69131.96137.75139.91148.83154.24161.80169.86171.80179.92187.69189.62

Age(ka)

0.6066.371

13.70319.85427.29831.81034.93339.40845.06451.67561.21372.23478.49593.22399.976

106.675112.862115.170123.206125.759131.440138.699140.446147.759154.757156.495

TOC(%)

.82

.82

.70

.75

.66

.591.381.411.411.801.122.181.981.932.572.032.193.182.251.731.551.691.321.381.791.47

CaCO,(%)

5.99.37.0

10.47.96.1

13.27.36.79.86.16.88.13.66.15.7:

8.26.7

13.27.58.59.19.76.98.3

Total sulfur(%)

0.641.451.050.330.910.410.730.360.100.740.101.870.451.111.060.930.720.810.591.320.440.801.030.231.891.22

Hydrogen index(mg HC/g TOC)

123

158

121

166

160

162

192

120

142157

Extract yield(mg/g TOC)

7249

10284757579

11616169

114889681596982665669707077818478

U \7 /SST(°C)

0.43/11.7

0.39/10.5

0.41/11.1

Note: * Void-corrected depth from Merrill and Rack (this volume).

were silylated with trimethylsilyltrifluoroacetamide (MSTFA). Ele-mental sulfur, which elutes in the aliphatic hydrocarbon fraction, wasremoved by treatment with copper filings.

Gas chromatography was performed on a Hewlett Packard 5890series II instrument equipped with a Gerstel temperature-pro-grammed cold injection system and a fused silica capillary column(Column A, aliphatic hydrocarbons: length = 30 m, inner diameter =0.25 mm, coated with SE-54, film thickness = 0.25 µm; Column B,polar fractions: length = 25 m, inner diameter = 0.22 mm, coated withsiloxane-carborane copolymer, film thickness = 0.1 µm). Helium wasused as the carrier gas, and the temperature was programmed from60°C (2 min) to 300°C (50 min) at a rate of 3°C/min. For the deter-mination of the U^-alkenone index the temperature was pro-grammed from 60°C (2 min) to 270°C at a rate of 10°C/min and thento 300°C at a rate of l°C/min (final hold time). GC/MS studies wereperformed with the same type of gas chromatograph equipped withcapillary Column A under the temperature conditions given aboveand coupled to a Finnigan SSQ 710 B mass spectrometer operated at70 eV.

Compound identifications are based on comparisons of relativegas chromatographic retention times and mass spectra with those re-ported in the literature. Quantifications were performed relative tothe amount of internal standards. Concentrations of alcohols (detect-ed as TMS ethers) were calculated from their ratios to the injectionstandard behenic acid methyl ester (BAME).

RESULTS AND DISCUSSION

Bulk Characteristics

The bulk data of elemental analysis, Rock-Eval pyrolysis, and ex-tract yields are summarized in Table 1. The table also includes thevoid-corrected depth for the samples studied (Merrill and Rack, thisvolume) and the interpolated absolute ages based on AMS 14C chro-nology and stable-oxygen-isotope stratigraphy (Ingram and Kennett,this volume).

In general, TOC values vary between 1.3% and 3.2% (percent byweight of dry sediment) with the highest concentration in a laminatedsample at 139.91 meters below seafloor (mbsf) (Sample 146-893A-

16H-1, 55-59 cm), near the boundary of isotopic Stages 5e and 5d. Inview of the elevated organic carbon contents, the hydrogen indicesare conspicuously low, with values varying between 120 and 192 mgHC/g TOC. However, the TOC concentrations and hydrogen indicesdetermined in this study fit well into the high-resolution concentra-tion profiles reported and discussed in detail by Stein and Rack (thisvolume).

Total sulfur (TS) contents range from 0.1% to 1.9%, and there isno evidence for a trend in concentration profile that corresponds toother bulk organic geochemical or lithologic characteristics. At thisstage, TS data are difficult to interpret without any knowledge aboutthe chemical state of the sulfur, which may occur as sulfide, sulfate,elemental sulfur, and bound into organic matter. The carbonate con-tents (reported as percentage of calcium carbonate) generally rangefrom 5% to 10% in most of the samples.

Most of the extract yields range between 60 and 100 mg/g TOC.Some higher values (116, 161, and 114 mg/g TOC, respectively)were observed for three samples between 57.78 and 86.00 mbsf(Samples 146-893A-7H-5, 22-27 cm; 146-893A-8H-3, 75-80 cm;and 146-893A-10H-3, 65-70 cm). All reported extract yields includeaverage amounts of about 2.5 mg ( 6 mg/g TOC) of elemental sulfur,which is soluble in the organic solvent used. For a recent or subrecentsediment, typical values of the extract yields are about 15 mg/g TOCor less (e.g., data obtained for deep-sea sediments from the CaliforniaBorderland, recovered during Deep Sea Drilling Project (DSDP) Leg63; Rullkötter et al., 1981). High bacterial activity may result in ele-vated extract yields, as is known from lacustrine systems like theMessel oil shale (Rullkötter et al., 1988). Otherwise, elevated extractyields may indicate the presence of migrated oil.

The relative distributions of the gross chromatographic fractions,obtained by separation of the total extracts, are presented in Figure 3.This figure shows the extract composition as percentages of eachfraction normalized to the total extract. Each of the extracts is domi-nated by the NSO fraction, which ranges from 30% to 63%. The por-tion of asphaltenes varies between 13% and 38% of the total extract.Laminated sediments tend to contain slightly more asphaltenes thannonlaminated sediments. The aliphatic hydrocarbons represent 5% to10% of the total extracts, whereas the amount of aromatic hydrocar-bons varies from 1 % to 4% with a trend of slight increase with depth.


70

60

50

g 4075o

o 30

o•Ea. 20

10

2/310.5

5/541.5

7/557.8

11/4 14/4 15/6 17/4 19/3 21/1 21/397.3 125.7137.8 154.2 171.8187.7189.6

Sample (core/section) and depth (mbsf)

•mi

mBD

Aliphatic hydrocarbons

Aromatic hydrocarbons

Asphaltenes

NSO compounds

Elemental sulfur

Figure 3. Yields of gross fractionation of total extracts of selected samples.

Elemental sulfur was present in all samples studied so far. The con-tent varied between 2% and 27% with the highest percentage ob-served in Sample 146-893A-7H-5, 22-27 cm. In Sample 146-893A-2H-3, 109-113 cm, the amount of elemental sulfur was below thegravimetric detection limit. With the exception mentioned for the as-phaltenes, there is no evidence for a significant relationship betweenbulk extract composition and lithology.

Molecular Investigations

Aliphatic Hydrocarbons

Figure 4 shows the gas chromatogram of the aliphatic hydrocar-bon fraction of Sample 146-893A-5H-5, 113-118 cm, which is rep-resentative of the gas chromatograms of all investigated aliphatichydrocarbon fractions. A major unresolved complex mixture (UCM)is common to all gas chromatograms, indicating bacterial degrada-tion of hydrocarbons (Milner et al., 1977). The n-alkane distributionpatterns are very similar for all samples studied and are typical for asupply from cuticular waxes of higher land plants (Eglinton andHamilton, 1967). The π-alkanes maximize at n-C29H60 in nine of thesamples and at n-C 3 1H 6 4 in Sample 146-893A-17H-4, 70-75 cm.Carbon preference index (CPI) values (Bray and Evans, 1961; Hunt,1979) of π-alkanes in the carbon atom number range from 25 to 33vary between 2.9 and 5.8.

Two phytene double bond isomers, phyt-1-ene and phyt-2-ene,were observed in all investigated hydrocarbon fractions. The summa-rized phytene concentration ranges from 2.5 to 15 µg/g TOC, with

40 60 80

Retention time (min)

100

Figure 4. Gas chromatogram of the aliphatic hydrocarbon fraction of Sample146-893A-11H-4, 96-101 cm. Odd-numbered «-alkanes are marked withtheir carbon numbers. Also marked are the standards used for quantification(BAME = behenic acid methyl ester; Sq = squalane). Pr = pristane, Ph = phy-tane, P = phyt-1-ene and phyt-2-ene, X = contaminant.

slightly higher concentrations of phyt-1-ene in each sample. Theywere identified by comparison of their mass spectra with literaturedata (Urbach and Stark, 1975) and relative retention times (Ikan et al.,1975). Most likely, these phytenes are diagenetic conversion prod-ucts of phytol (Volkman and Maxwell, 1986; Didyk et al., 1978),which is derived from planktonic chlorophyll a by ester cleavage;methanogenic bacteria may be an additional source of phytenes(Brasselletal., 1981).

Except for the phytenes, compounds of autochthonous origin arepresent only in trace amounts in the aliphatic hydrocarbon fraction.Examples are series of sterenes and steradienes, which are derivedfrom steroid alcohols by diagenetic transformations (de Leeuw et al.,1989; see also "Steroidal Alcohols" section in this paper for sourcesof steroid compounds) and from pentacyclic triterpenes like bacterialfernenes and diploptene derived from bacteria, respectively (Ouris-sonetal., 1979; Howard, 1980; Brassell et al., 1981; Brassell and Eg-linton, 1983). However, the fact that sterenes and steradienes arepresent only in trace amounts emphasizes the very early diageneticstage of the organic material. Therefore, the analysis of polar com-pounds such as fatty acids, alcohols, and ketones is appropriate for amore detailed assessment of origin and paleoenvironmental condi-tions of deposition of that part of the organic matter that is represent-ed by the extractable fraction.

Migrated Hydrocarbons

An additional organic matter source of Santa Barbara Basin sedi-ments is documented especially in the aliphatic hydrocarbon fractionby the presence of petroleum hydrocarbons as has been observed ear-lier for a wide range of shallow sediments in the California Bight bySimoneit and Kaplan (1980). All investigated aliphatic hydrocarbonfractions of sediments from Site 893 contain compound series thattypically occur in sediments with mature organic matter or crude oil.The petroleum-type compound series comprise steranes and diaster-anes as mixtures of stereoisomers characteristic of thermally maturedorganic matter, of tricyclic terpanes, and of hopanes with the thermal-ly most stable 17α(H),21ß(H) configuration (Seifert and Moldowan,1980; Simoneit and Kaplan, 1980). Compound identification is basedon relative retention times and comparison of mass spectra with thosereported in the literature (e.g., Peters and Moldowan, 1993; Rullköt-teretal., 1984, 1987).


steroid alcohols


Figure 5. Reconstructed ion chromatogram and mass chromatogram of frag-

ment m/z 191 from GC/MS analysis of Sample 146-893A-21H-3, 7-12 cm.

The petroleum-type terpanes are shown in black. Compounds identified are

listed in Table 2. The n-alkanes (numbers) and the internal standard (SQ =

squalane) are marked in the RIC trace.

As an example, Figure 5 shows the reconstructed ion chromato-gram (RIC) together with the mass chromatogram of fragment m/z191 for the aliphatic hydrocarbon fraction of Sample 146-893A-21H-3, 7-12 cm. This mass spectrometric fragment is characteristic of tri-and pentacyclic terpanes. Figure 5 illustrates the dominance of petro-leum-derived hopanes and tricyclic terpanes among the polycyclicsaturated hydrocarbons (marked components are listed in Table 2).All investigated hydrocarbon fractions contain 28,30-^wor-hopane,which is a characteristic component in oils from the Californian

Table 2. Terpenoid compounds from migrated oil detected in the ali-

phatic hydrocarbon fractions of Santa Barbara Basin sediments.*

Symbol(Fig- 5)

ABCDEFGHIJKLMNDNHOPQRSTUV

wXYZA'B'CD'

Note: *cf.

Compound

C22-tricycIic teΦaneC2rtricyclic teΦaneC2rtricyclic teΦaneC24-tricyclic teΦaneC25-tricyclic teΦaneCi6-tricyclic teΦaneC-,6-tricyclic teΦaneC2S-tricyclic terpaneC-,8-tricyclic teΦaneC29-tricyclic teΦaneC79-tricyclic teΦane25, 28, 30-/W/ior-17α(H)-hopane22, 29, 30-mnor-17α(H)-hopaneC^-tricyclic teΦane28, 30-dinor-\la(H)-hopane30-«or-17α(H), 21ß(H)-hopane30-fto/--moretane18α(H)-oleanane17α(H), 21ß(H)-hopaneMoretane17α(H), 21ß(H)-(22S)-Aomσ-hopane17α(H), 21 ß(H)-(22R)-/jomo-hopane17α(H), 21ß(H)-(22S)-rf(/jomo-hopane17α(H),21ß(H)-(22R)-d(7iomo-hopane17ß(H), 21ß(H)-Ao/no-hopane17α(H), 21 ß(H)-(22S)-/π7;omo-hopane17α(H), 21 ß(H)-(22R)-fW/jomo-hopane17α(H), 21 ß(H)-(22S)-tetrakishomo-hopane17α(H), 21 ß(H)-(22R)-tetrakishomo-hop<me17α(H), 21 ß(H)-(22S)-pentakishomo-hopam17α(H), 21ß(H)-(22R)-/7é•«toteAomo-hopane

Figure 5.

40 60 80


100

Figure 6. Gas chromatogram of the alcohol fraction of the nonlaminatedSample 146-893A-21H-3, 7-12 cm. The most abundant compound, 5α(H)-cholestan-3ß-ol (m) and the injection standard (BAME), are labeled.

Monterey Formation (Curiale et al., 1985) and is common in surfacesediments of the California Bight as an indicator of weathered petro-leum-type material (Simoneit and Kaplan, 1980; McEvoy et al.,1981; Rullkötteretal., 1981; Simoneit and Mazurek, 1981). Its con-centration in the Santa Barbara Basin sediments from Hole 893Aranges from 10 to 42 µg/g TOC. Considering that the offshore Cali-fornia area is a major petroleum generation and production area withMonterey Formation source rocks outcropping on the coast and in theCalifornia mainland and with numerous petroleum seeps onshore andoffshore, it is reasonable to assume that the observed petroleum hy-drocarbons were supplied to the Santa Barbara Basin as part of erod-ed material. It is also possible—but considered less likely—thatpetroleum has locally migrated upward from fractured Montereyrocks to the subsurface and have impregnated the shallower SantaBarbara Basin sediments. The n-alkanes apparently were microbiallydegraded, so the unresolved complex mixtures and the petroleum-type polycyclic hydrocarbons are the only evidence for the supply ofmature organic matter in the aliphatic hydrocarbon fractions.

n-Alcohols

A typical gas chromatogram of the alcohol fraction of a nonlami-nated sediment sample (146-893A-21H-3, 7-12 cm), which demon-strates the predominance of a complex mixture of steroidal alcoholsin this fraction, is presented in Figure 6. All alcohols were analyzedas their TMS ethers. Long-chain n-alcohols are not present in thisfraction. In the liquid chromatographic separation procedure used,long-chain n-alcohols (with more than 20 carbon atoms) eluted in theketone fraction whereas the more polar n-alcohols with less than 20carbon atoms eluted in the alcohol fraction. However, in all investi-gated alcohol fractions, n-alcohols with less than 20 carbon atoms areonly present in minor amounts with concentrations less than 1 µg/gTOC. The long-chain n-alcohols show a marked preference of evencarbon atom number homologs. All odd-numbered n-alcohol concen-trations are near the detection limit. Figure 7 compares the long-chainn-alcohol concentrations of Samples 146-893A-5H-5, 113-118 cm(nonlaminated), 146-893A-11H-4, 96-101 cm (laminated), and 146-893A-21H-3, 7-12 cm (nonlaminated). The most abundant n-alcoholin the nonlaminated samples is n-tetracosanol, followed by n-octa-cosanol, whereas the situation in the laminated sample is reversed.The n-alcohol concentrations of the two nonlaminated samples are

206


üO

60

50

40

w 30co"•sc<Ducoü

20

10

21 22 23 24 25 26 27 28 29 30 31

Number of carbon atoms

• 146-893A-5H-5, 113-118 cm; depth: 41.52 mbsfI I 146-893A-11H-4, 96-101 cm; depth: 97.26 mbsf• 146-893A-21H-3, 7-12 cm; depth: 189.62 mbsf

Figure 7. Distribution and concentrations of long-chain n-alcohols (C22 toC30) in Samples 146-893A-5H-5, 113-118 cm (nonlaminated); 146-893A-11H-4, 96-101 cm (laminated); and 146-893A-21H-3, 7-12 cm (nonlami-nated).

about twice as high as those in the laminated sample, probably indi-cating a reduced terrigenous organic matter supply during sea-levelhighstands.

Steroidal Alcohols

In Figure 8, the reconstructed ion chromatograms show the elu-tion range of steroid alcohols for three selected Samples 146-893 A-5H-5, 113-118 cm (nonlaminated); 146-893A-11H-4, 96-101 cm(laminated); and 146-893A-21H-3, 7-12 cm (nonlaminated). Com-pound identification, as compiled in Table 3, is based on relative re-tention times and comparison with mass spectra from the literature(Budzikiewicz, 1972; Brassell, 1980; McEvoy, 1983; Wardroper,1979; Wyllie and Djerassi, 1968). Quantification of major compo-nents is based on FID response in the gas chromatograms. We iden-tified about 40 different steroid alcohols with carbon numbersranging from 26 to 30 and different numbers and positions of doublebonds as well as different side-chain substituents at C-24. This com-plexity is characteristic for a steroid origin from a variety of marineorganisms (Wardroper et al, 1978; Brassell, 1980). Except for dino-sterol, 4-methylsterols are present only in minor amounts. The steroldistributions in the Santa Barbara Basin samples are largely similarto the distribution reported for a Quaternary sample from the nearbyDSDP Site 467, San Miguel Gap (McEvoy et al., 1981).

With a few exceptions, the identified compounds are present ineach investigated sample. Nevertheless, the samples show consider-able differences in steroid distribution patterns. The concentrations of




–̂Figure 8. Partial reconstructed ion chromatograms of the alcohol fractions ofthree selected samples showing the elution range of steroid alcohols. Thelabeled identified compounds are listed in Table 3. A. Sample 146-893A-5H-5, 113-118 cm (nonlaminated). B. Sample 146-893A-11H-4, 96-101 cm(laminated). C. Sample 146-893A-21H-3, 7-12 cm (nonlaminated).

individual components in the laminated Sample 146-893A-11H-4,96-101 cm, are noticeably lower than those in the two nonlaminatedsamples. In the laminated sample, dinosterol is the major steroidal al-cohol, corresponding to the present-day situation in surface-near sed-iments in the center of the basin as reported by Kennedy and Brassell(1992b). Dinosterol is well known as a biosynthetic product of di-noflagellates (Boon et al., 1979), although in more recent studies oth-er marine sources have been identified (Volkman et al., 1993).

207


Table 3. Steroid alcohols identified in selected samples, Hole 893A.*

Symbol(Fig. 8) Compound 5H-5

5.322.2

3.2

8.911.4

15.934.')38.0

63.674.768.248.')

Concentration(µg/g TOO

11H-4

1.16.81.7

4.61.0

3.811.79.5

29.533.539.926.6

21H-3

4.920.48.2

16.55.9

15.623.648.8

43.5138.936.762.7

a 24-w;/-cholesta-5,22(E)-dien-3ß-olb 24-«r;r-5α(H)-cholesten-22(E)-3ß-olc 24-/!r;r-5α(H)-cholestan-3ß-old 5ß(H)-cholestan-3ß-ol + 24-/zo/--cholesta-4, 22-dien-3-one ?e 5ß(H)-cholestan-3-olf 27-/!w-24-methylcholesta-5, 22(E)-dien-3ß-ol** ?g Unknown compound (only present in Sample 146-893A-11H-4, 96-101 cm)h 27-no/ -24-methyl-5a(H)-cholest-22(E)-en-3ß-ol** ?i Cholesta-5,22(E)-dien-3ß-olj 5α(H)-cholest-22(E)-en-3ß-olk Unknown C^-stanol1 Cholest-5-en-3ß-ol

m 5α(H)-cholestan-3ß-oln 24-methylcholesta-5,22-dien-3ß-ol0 24-methyl-5α(H)-cholest-22(E)-en-3ß-olp Unknown C27-cholestenolq 24-methyl-5α(H)-cholest-24(28)-en-3ß-ol possibly coelutes with 24-methylcholest-5-en-3ß-olr 24-methyl-5α(H)-cholestan-3ß-ols 23, 24-dimethylcholesta-5, 22-dien-3ß-olt 23,24-dimethyl-5α(H)-cholest-22(E)-en-3ß-olu + 24-ethylcholesta-5, 22(E)-dien-3ß-olv 24-methylcholesta-4,22-dien-3-onew + 24-ethyl-5α(H)-cholest-22(E)-en-3ß-olx 24-ethylcholest-5-en-3ß-oly + 23, 24-dimethyl-5(X(H)-cholestan-3ß-ol (in trace amounts)z Unknown C^-steratrienolA 24-ethyl-5α(H)-cholestan-3ß-olB + 24-ethylcholesta-5, 24(28)-dien-3ß-olC +4,24-dimethyl-5α(H)-cholestan-3ß-olD 4, 23, 24-trimethylcholest-22-en-3ß-ol (dinosterol)E 24-propylcholest-5,24-dien-3ß-olF 24-ethyl-5α(H)-cholest-7-en-3ß-olG 24-propylcholest-5,24(28)-dien-3ß-olH + Cw-4-methylstenol1 + 4, 23, 24-trimethy!-5α(H)-cholest-7-en-3ß-olJ Unknown CjQ-steradienolK +4, 23, 24-trimethyl-5α(H)-cholestan-3ß-olL 4, 23, 24-trimethyl-5α(H)-cholestan-3ß-ol, C-23 or C-24-epimer of K ?M Unknown C-.irsterenoneN Unknown CM-steradienolO Unknown compound

27.2

62.7

19.3

3.8

31.5

42.4

7.9

73.0

50.8

10.0

Notes: *cf. Figure 8. **27-nor-24-methyl-sterol isomers were reported by McEvoy (1983). Question marks indicate tentative assignments. Absolute concentrations are included wherequantification was possible.

In contrast to this, the two nonlaminated samples contain 5α(H)-cholestan-3ß-ol (m) as the most abundant compound. This compoundis presumably derived, at least in part, from diagenetic reduction ofcholesterol (e.g., Mackenzie et al., 1982), but direct formation by ma-rine organisms has also been described in the literature (red algae:Chardon-Loriaux et al., 1976; sponges: Erdman and Thomson, 1972;benthic organisms: Ballantine et al., 1981). In the two shallower sam-ples, cholesterol (1) is present in a concentration similar to that of thesaturated analog, 5α(H)-cholestan-3ß-ol (m), whereas in the deepestsample the concentration of compound m is 3 times higher than thatof compound 1. This probably indicates the progress of diagenetictransformation of cholesterol by microbial reduction. Other typicaldiagenetic reactions of steroid alcohols (e.g., the formation of hydro-carbons by loss of the hydroxyl group at C-3) are of minor impor-tance in the investigated samples as it is indicated by the very lowconcentrations of sterenes in the aliphatic hydrocarbon fractions.

A more general way of comparing the steroid composition of dif-ferent samples is to investigate their carbon number distribution. Thisapproach has been introduced by Huang and Meinschein (1976,1979), who used this method to determine the source of organic mat-ter in near-surface sediments. It is based on the idea that C29 steroidalcohols are derived mainly from terrestrial higher plants. In fact, thisis valid for 24-ethylcholest-5-en-3ß-ol, which is the main sterol inhigher plants, but exceptions have been noted (e.g., for cyanobacte-ria; Volkman, 1986). In the case of the very complex steroid patternsof the Santa Barbara Basin sediments, we extended this approach toinclude mainly marine-derived C26 and C30 steroid alcohols in addi-

c50 T

111Carbon atom number

Figure 9. Carbon number distribution of steroid alcohols of three selectedsamples. The percentages are normalized to the total amount of steroid alco-hols. Quantitative data were obtained by comparison of peak intensity rela-tive to a known steroid concentration in the RIC. A. Sample 146-893A-5H-5,113-118 cm (nonlaminated). B. Sample 146-893A-11H-4, 96-101 cm (lami-nated). C. Sample 146-893A-21H-3, 7-12 cm (nonlaminated).

tion to the C27-C29 components. The results for three samples are re-ported in Figure 9. For this purpose, semiquantitative calculations,based on the RIC signal intensity relative to a known steroid concen-tration (5α(H)-cholestan-3ß-ol (m), from GC-FID measurements),were performed to obtain the summed concentrations for each carbonatom number. Both nonlaminated samples are characterized by sim-ilar carbon number distributions with a maximum at C27, whereas the


16 100

16:1

14

12

15

ER

18:1

18

20

Sq

22

RXJ

26

Sqe

24

23/

25

28

2730

29

40 60


80

Figure 10. Gas chromatogram of the fatty acid fraction of Sample 146-893A-21H-3, 7-12 cm. The rc-fatty acids are labeled with their carbon atom num-ber. In addition, the internal standard (ER = erucic acid), the injection stan-dard (Sq = squalane), and squalene (Sqe) are marked. The occurrence ofsqualene in this fraction is thought to result from chemical transformation ofa more polar precursor during separation.

distribution of the laminated sample is characterized by a relativelyuniform carbon number distribution from C27 to C3 0 with a weak max-imum at C27. An evaluation of steroid alcohol sources in terms of ma-rine vs. terrestrial origin, based on the carbon number distributions,does not show significant differences among the samples, becausethe relative abundance of the predominantly terrigenous C29 sterolsshows little variation. Thus, the different distributions most likelyrepresent different assemblages of marine source organisms in theSanta Barbara Basin.

The summarized sterol concentration in the laminated Sample146-893A-11H-4, 96-101 cm, is noticeably lower (360 µg/g TOC)than that of the two nonlaminated Samples 146-893A-5H-5,113-118cm (640 µg/g TOC) and 146-893A-21H-03, 7-12 cm (850 µg/gTOC). The C3 0 steroids, which are of a marine origin, have a signifi-cantly higher concentration in the laminated sample.

n-Fatty Acids

A gas chromatogram of a representative fatty acid fraction (Sam-ple 146-893A-21H-3, 7-12 cm) is shown in Figure 10. In all investi-gated samples, we observed a bimodal distribution of n-fatty acidswith a strong predominance of even carbon number homologs, max-imized at C1 6 and C26. Similar fatty acid distributions were observedin a study of lipid composition (Simoneit and Mazurek, 1981) in twoQuaternary samples from the nearby Site 467. Monounsaturated ac-ids occur in the carbon number range from 14 to 18, with a maximumat C16 :1. The long-chain fatty acids ( C22) reflect a contribution of ter-rigenous higher plants (Kolattukudy, 1976) whereas the saturated andunsaturated fatty acids in the lower carbon number range are presum-ably derived mainly from algae (Cranwell, 1974). Figure 11 displaysthe fatty acid distribution and the concentrations of individual com-pounds, normalized to TOC, for three selected samples (146-893A-5H-5, 113-118 cm, nonlaminated; 146-893A-11H-4, 96-101 cm,laminated; and 146-893A-21H-3, 7-12 cm, nonlaminated). In thesesamples, n-Cl&0 is the major compound in the acid fraction with con-centrations ranging from 61 to 81 µg/g TOC, followed by π-C26:0 inthe nonlaminated Samples 146-893A-5H-5, 113-118 cm, and 146-893A-21H-3, 7-12 cm, and «-C16:1 in the laminated Sample 146-893A-11H-4, 96-101 cm. In contrast to the nonlaminated samples,the latter sample contains higher plant-derived long-chain fatty acidsin lower concentrations (concentration of n-C26:0: 21 µg/g TOC vs. 59

S 20

. Jl12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Carbon atom number

1 0 0 •

8 8 0•

1 6 0 •

I 40"S 2 0 .

oo . Jj l l

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Carbon atom number

100

O 80

3 60-coS 40-

Së 20-

• •lπl l • l • l11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Carbon atom number

Saturated n-fatty acids EH Monounsaturated n-fatty acids

Figure 11. Distribution of saturated and monounsaturated n-fatty acids inthree selected samples. A. Sample 146-893A-5H-5, 113-118 cm (nonlami-nated). B. Sample 146-893A-11H-4, 96-101 cm (laminated). C. Sample146-893A-21H-3, 7-12 cm (nonlaminated).

and 32 µg/g TOC in the nonlaminated samples). This is similar to thesituation observed for the concentrations of higher plant-derived n-alcohols and of sterols.

Ketones

We restricted our preliminary investigation of selected ketonefractions to the determination of the alkenone unsaturation index(U 37), which allows the calculation of proximate paleo-sea-surfacetemperatures (SST, Brassell et al., 1986; Prahl and Wakeham, 1987;Prahl et al., 1988). The values are reported in Table 1. In the sedi-ments from Hole 893A, C3 7 : 3 and C37;2 alkenones were detected in lowconcentrations in the range of 2 to 5 µg/g TOC for the sum of bothcompounds. This is in contrast to results of Kennedy and Brassell(1992b), who investigated near-surface sediments of the 20th centuryfrom the central Santa Barbara Basin. In their study, the C37 alkenoneconcentrations are between one and two orders of magnitude higher


than ours although the concentrations of selected steroid alcoholswere at similar levels. This may indicate a selective degradation ofthe unsaturated long-chain alkenones or a significant increase of thebiomass of main producers, especially the coccolithophorid Emilian-ia huxleyi (Marlowe et al., 1984).

Using the equation of Prahl and Wakeham (1987), the U \η data inTable 1 result in estimated SST values of 10.5 to 11.7°C, with thelowest temperature in the laminated Sample 146-893A-11H-4, 96-101 cm. Averaged alkenone SST values for the Santa Barbara Basinbased on sediments of this century as determined by Kennedy andBrassell (1992b) were just below 14°C.

CONCLUSIONS

Sediments in the Santa Barbara Basin, spanning the age range ofthe last 150,000 yr, are enriched in organic carbon. This reflects anintense primary marine bioproductivity as well as the supply of high-er land-plant debris from the neighboring continent and of mature or-ganic matter from the erosion of Monterey Formation petroleumsource rocks or oil seeps. Local oil migration from greater depthscannot be excluded but is considered less likely. Relatively low hy-drogen indices are consistent with a major fraction of refractory or-ganic matter in the sediments, which may be due to oxidation of theterrigenous organic matter during transport and/or to extensive mi-crobial reworking after deposition of marine and terrigenous organicmatter. Elevated total sulfur contents and free elemental sulfur in theextracts indicate that the sediments were reducing with intense mi-crobial sulfate reduction ongoing. Abundant methane in the coresshows that the microbial system extended into the methanogenesiszone. Thus, although the depositional conditions obviously were fa-vorable for organic matter preservation throughout the time span cov-ered, postdepositional microbial activity is likely to have converted amajor portion of the labile (marine) organic matter into more refrac-tory material.

The aliphatic hydrocarbon fractions of all sediments studied onthe molecular organic matter level show clear indications of the pres-ence of mature hydrocarbons that were bacterially altered either be-fore or after impregnation of the shallow sediments. The distributionpattern of the pentacyclic triterpanes indicates that the migrated hy-drocarbons were thermally generated in the Miocene Monterey For-mation, which is an active petroleum source rock in this area. Thealiphatic hydrocarbon fractions represent only a small amount of theextractable organic matter, but as common for biodegraded oils therewill also be a significant amount of this type of allochthonous organicmatter in the more polar extract fractions (NSO fraction, asphalt-enes). Because these compounds are difficult to identify on a molec-ular level, it is difficult to estimate the total amount of biodegradedpetroleum-type material in the Santa Barbara Basin sediments at Site893. The high total extract yields may result both from this petroleumfraction and the intense activity of bacteria probably also thriving onthe autochthonous primary marine organic matter.

Molecular indicators for the presence of terrigenous organic mat-ter were found in all extract fractions. They range from long-chain n-alkanes to fatty acids and alcohols with similarly long straight chains.The C29 steroids may at least in part also be derived from terrestrialsources.

The alcohol fractions are dominated by sterols and stanols, mostof which likely originated from the remnants of marine organisms.Dinosterol, probably from dinoflagellates as the major source, is themost abundant sterol in the laminated sample studied but occurs to-gether with a series of steroids of similarly high concentrations. In thetwo nonlaminated samples studied so far, cholestan-3ß-ol is the mostabundant single component, and dinosterol has a low relative concen-tration. This may indicate a change of primary productivity pattern

with time, but this will have to be substantiated in the subsequentanalysis of a greater number of samples.

Concentrations of long-chain alkenones in the Hole 893A sedi-ments are much lower than those in surface sediments covering the20th century. This again may reflect a change in primary productivitypattern (i.e., prymnesiophytes are more abundant in the most recenttimes), or a diagenetic destruction of these compounds. Because thedouble-bond homology is known not to change by this process, thedetermination of paleo-sea-surface temperatures should still be valid.The values determined for three samples show little variation withtime, but temperatures calculated this way are 2°-3°C lower thanthose determined for surface sediments. The significance of this dif-ference is not clear at this moment.

ACKNOWLEDGMENTS

We are indebted to Dr. Barbara Scholz-Böttcher (ICBM) for sup-port in GC-MS analysis. We are grateful to Drs. B.R.T. Simoneit (Or-egon State University, Corvallis) and M. Lyle (Boise StateUniversity) for thorough reviews of the initial manuscript and helpfulsuggestions to improve it. The study was financially supported by theDeutsche Forschungsgemeinschaft (DFG), grant no. Ru 458/8.

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Date of initial receipt: 15 September 1994Date of acceptance: 23 March 1995Ms 146SR-307

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14. PRELIMINARY ASSESSMENT OF ORGANIC ...PRELIMINARY ASSESSMENT OF ORGANIC GEOCHEMICAL SIGNALS IN SEDIMENTS FROM HOLE 893A, SANTA BARBARA BASIN, OFFSHORE CALIFORNIA 1 K.-U. Hinrichs,